given a seed value "iv" (optional), returns a deterministic string of pseudo-random hex values of length "chars"

rc4stream.js

function rc4stream(chars, iv) {
	var s = [], key=iv || (Date.now() / (performance.now()*1000)).toString(36) + (chars/Math.random()),
		j = 0,
		x, res = [],
		i, j, y, l = key.length,
		m = Math.ceil(chars/2);
	for(i = 0; i < 256; i++) s[i] = i;

	for(i = 0; i < 256; i++) {
		j = (j + s[i] + key.charCodeAt(i % l)) % 256;
		x = s[i];
		s[i] = s[j];
		s[j] = x;
	}
	for(y = 0, i = 0, j = 0; y < m; y++) {
		j = (j + s[i = (i + 1) % 256]) % 256;
		x = s[i];
		s[i] = s[j];
		s[j] = x;
		res.push( (0+(80 ^ s[(s[i] + s[j]) % 256]).toString(16)).slice(-2) );
	}
	return res.join("").slice(0, chars);
}

//ex:
// rc4stream(500, "this is not a great iv");